Rechargeable batteries manufactured from laminates of solid polymer electrolytes and sheet-like electrodes display many advantages over conventional liquid electrolytes batteries. These advantages include: lower overall battery weight, high power density, high specific energy, and longer service life. In addition, they are more environmentally friendly since the danger of spilling toxic liquid into the environment is eliminated.
Solid polymer battery components generally include: positive electrodes (also referred to as cathodes), negative electrodes (also referred to as anodes), and an insulating material capable of permitting ionic conductivity, such as a solid polymer electrolyte, sandwiched therebetween. The anodes are usually made of light-weight metals foils, such as alkali metals and alloys thereof typically lithium metal, lithium oxide, lithium-aluminum alloys and the like. The composite cathodes are usually formed of a mixture of active material such as a transitional metal oxide, an electrically conductive filler, usually carbon particles, an ionically conductive polymer electrolyte material and a current collector usually a thin sheet of aluminum.
Composite cathode films are usually obtained by coating onto a current collector a mixture of a solvent and cathode materials with a doctor blade, for instance, and evaporating the solvent. This process is inefficient for the mass production of cathode films and results in cathode films having a relatively high porosity, and therefore decreased density.
Since solid polymer electrolytes are usually less conductive than liquid polymer electrolytes, solid or dry electrochemical cells must be prepared from very thin films (total thickness of approximately 35 to 250 microns) to compensate for the lower conductivity, with a high film contact surfaces and provide electrochemical cells with high power density. Solid cathode films must therefore be produced into very thin films of generally ranging from about 35 to 125 microns.
One of the most efficient manufacturing processes for obtaining thin sheets is the process of continuous extrusion. U.S. Pat. No. 5,725,822 to Keller et al. discloses a method for extruding electrode material by liquid injection. The solid particulate of active electrode materials are partially mixed with a minor portion of the components of the polymer electrolyte and fed into a first feed throat of the extruder while the remaining polymer electrolyte composite, preferably rich in liquid components including at least one solvent, is fed downstream through a second feed throat. The process has been found to provide a composite having a high ratio of solid active material electrode/electrolyte and by separately mixing the components, the electrode composition may be adjusted to provide optimal proportions of all materials for a given application. However, this process is limited to polymer electrolyte binders capable of withstanding the extrusion processing conditions, in particular the temperature, pressure and shear conditions such as polyacrylonitrile (PAN), polyvinylidene difluoride (PVDF), polyvinylpyrrolidone (PVP) and the like mixed with a solvent. The use of solvents in the extruded mixture results in a thicker composite cathode sheet that displays a high porosity and a rough surface finish. The latter characteristics are generally detrimental to the efficiency of the electrochemical cell produced.
U.S. Pat. No. 5,316,556 to Morris also discloses an apparatus and method for extruding a cathode in which the cathode material is mixed to an homogenous state, and then transported under constant or increasing shear stress to a point of extrusion such that it is extruded at constant rate. The cathode material disclosed is referred to as a shear thinning material as it exhibits non-Newtonian fluid characteristics; that is its viscosity decreases as the material is subjected to increasing shear stress. The solution proposed to transport the melted ‘shear thinning’ cathode material smoothly to the extruder exit nozzle is simply to maintain a minimum pressure and therefore a minimum amount of shear stress on the ‘shear thinning’ cathode material to ensure that the viscosity or flow resistance of the cathode material remains below a certain value to prevent blockage of the cathode material in the extruder. Experience has shown however that such a simple technique is inadequate for a wide variety of cathode materials, especially when the solid content of the cathode material is above 30% by weight.
Cathode materials having a high solid content of active cathodic material and conductive filler (above 30%) like polymers of the polyether family such as polyethylene oxide having a high percentage of solid particles of vanadium oxide and carbon cannot withstand normal extrusion conditions and, more particularly, high temperatures and high shear conditions. Polyethers have a low melting point (around 50° C.) and are chemically unstable under extrusion conditions thereby making them extremely difficult to process through an extruder to form a thin positive electrode composite sheet. Neither Keller et al. nor Morris provide a viable process for extruding cathode thin films made of a polyether binder having a high percentage of solids.
Thus there is a need for a solid polymer electrolyte-cathode sheet having a high solid content which can be extruded and a method for extruding a cathode sheet having a high solid content.